4 El subjuntivo con expresiones de duda o negaciónWhen creer and estar seguro/a express certainty, they are followed by the indicative.El guía está seguro de/ cree/ piensa que podemos navegar los rápidos.The guide is sure/believes/thinks that we can run the rapids.However, when creer and estar seguro ask a question that expresses the speaker’s doubt or uncertainty, they are followed by the subjunctive.¿Está seguro/ cree/ piensa el guía que podamos navegar lor rápidos?Is the guide sure/ Does the guide believe/ think that we can run the rapids?

7 El presente perfecto del subjuntivoThe present perfect subjunctive is used to express reactions to events that have occurred in the past but are closely tied to the present.Espero que hayan visto las cataratas.I hope that they have seen the waterfalls.Dudo que hayan cruzado el río.I doubt that they have crossed the river.